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OSD LOCKED/UNLOCKED This function allows you to lock the current control settings, so that they cannot be inadvertently changed. Press and hold the MENU button for several seconds. The message "OSD LOCKED" should appear. You can unlock the OSD controls at any time by pushing the MENU button for several seconds. The message "OSD UNLOCKED" should appear.

OSD Lock & UnlockWhen pressing the AUTO button after locking the OSDWhen pressing the MENU button after locking the OSDThis is the function that locks the OSD in order to keep the current states of settings or prevent othersfrom adjusting the current settings.Lock : Hold down the MENU button for more than five (5) seconds to activate the OSD adjustmentlock function.Unlock : Hold down the MENU button for more than five (5) seconds to deactivate the OSD adjustmentlock function.

The following site has step by step instructions on steps you can try: http://www.ehow.com/how_5008634_fix-lockout-display-computer-monitor.html
I'll paste them below:
All computer monitors, whether they are LCD or CRT style, have a built-in feature called the on screen display (OSD) lock out that prevents users from viewing any images on the monitor. If your computer screen has accidentally been set to the OSD lock out setting, there are several different ways you can remove the lock out, depending on the make and model of your particular computer monitor.1 Power your computer completely off and then remove the monitor cable from the port on the back side of the computer case. Reattach the cable and make sure the pins are correctly screwed in all of the way. Power the computer back on and check to see if the OSD lock out message is still being displayed.2 Check the manual that came with your computer or monitor and locate the troubleshooting section if you are still receiving the OSD lock out message. Find the OSD lock out heading and read the method that your particular model of monitor uses to remove the lock out. Follow the instructions to remove the lock out.3 Attempt generic lock out removal procedures if you no longer have the manual that came with your monitor. Press and hold the "Menu" button located at the bottom end of your monitor for 15 seconds and wait to see if the OSD lock out message disappears.4 Press the "Menu" button again if the lock out message remains on the screen. Use the arrow buttons to navigate through the main menu options. Highlight the "Settings" or "OSD Menu" option and then hit the Menu key again to access the new list of settings.5 Scroll down to the "OSD Lock" heading. Use the arrow keys to change the setting to "No." Hit the Menu key again to return to the main menu. Scroll over to the "Exit" setting and then hit the Menu key a final time to exit the menu setting and remove the OSD lock out.

The OSD can be enabled or disabled by pressing and holding the Menu button on the front panel for 10 seconds. If the OSD is locked, the warning message OSD Lock displays for ten seconds.

* If the OSD is locked, press and hold the Menu button for 10 seconds to unlock the OSD.
* If the OSD is unlocked, press and hold the Menu button for 10 seconds to lock the OSD.

OSD stands for "On Screen Display." It is an onscreen configuration menu included with various monitors.
When you press the MENU button or one of the other buttons on the
front or side of a monitor, the onscreen display may appear.
The OSD typically includes monitor calibration options, such as
brightness, contrast, and color adjustments. Some OSDs may also include
horizontal and vertical position controls as well as tilt and keystone
adjustments.
If you can't make adjustments to the monitor settings using the OSD
interface, the onscreen display may be locked. You may also see the
phrase "OSD Locked" appear when you try to change the settings. Some
monitors include an option within the menu called "OSD Lock", which you
can turn on or off. If this option is available, simply turn OSD Lock
off and you will be able to make adjustments to the settings.
If your monitor does not include the "OSD Lock" option, you may be
able to unlock the OSD by pressing and holding the MENU button on the
front or side of the monitor for several seconds. If you try this
method, be patient! It may take up to 15 seconds for the OSD to unlock.

The OSD can be enabled or disabled by pressing and holding the Menu button on the front panel for 10 seconds. If the OSD is locked, the warning message OSD Lock displays for ten seconds.

If the OSD is locked, press and hold the Menu button for 10 seconds to unlock the OSD.

If the OSD is unlocked, press and hold the Menu button for 10 seconds to lock the OSD

Or else you can try this method also,
Hold the menu/ok button for ten seconds, to unlock the OSD. If this doesn't solve the problem, then it's probably the button that got stuck. Remove the casing, locate the menu button's contact switch on the circuit board, and try to release it using something thin like a screwdriver, x-acto knife, or set of plyers.If this doesn't work, chances are the menu button is stuck on a loop, sending a continuous signal to the board (which keeps locking/unlocking the OSD).
If so, your only solution (other than take it to a repair shop) is to remove it, by prying it off with a screwdriver or set of plyers. Of course, you'll lose access to the OSD features, but I guess this is better than to have a message stuck on front of your screen all the time..

Locking OSD settings
When the OSD is locked, settings cannot be changed in the OSD menu.
The OSD functionality is enabled or disabled by pressing the Menu button and holding it for 10 seconds. Upon disabling the OSD, an OSD Lock-out message is displayed for 10 seconds. When the OSD is locked and an OSD menu button is pressed, the OSD Lock-out message is displayed for an additional 10 seconds. When the OSD is locked, pressing the OSD menu button for 10 seconds unlocks the OSD and displays the main menu.
Identifying monitor conditions
Special messages display on the monitor screen for the following monitor conditions:

Input Signal Out of Range : Indicates the monitor does not support the input signal because the resolution and/or refresh rate are set higher than the monitor supports. Recommended refresh rates are as follows:

No Input Signal : Indicates the monitor is not receiving a video signal from the computer on the monitor video input connector. Check whether the computer or input signal source is off or in the power-save mode.

Auto Adjustment in Progress : Indicates the auto-adjustment function is active and in process.

OSD Lock-out — The OSD can be enabled or disabled by pressing and holding
the Menu button on the front panel for 10 seconds. If the OSD is locked, the
warning message “OSD Lock-out” appears for 10 seconds.

If the OSD is
locked, press and hold the Menu button for 10 seconds to unlock the
OSD.

If the OSD is unlocked, press and hold the Menu button for 10
seconds to lock the OSD.